Well kiddo, have you ever heard the phrase "dress to impress"? Smart casual is a way of dressing in a nice and stylish way, but not too fancy, like you're going to a party or a wedding.
It's like being in the middle between looking super fancy or just wearing your regular everyday clothes, like you would wear to school or to the playground.
For boys, smart casual could mean wearing a nice pair of pants, like khakis or chinos, and a clean button-up shirt, but not necessarily a tie or a suit jacket. For girls, it could be something like a pretty blouse or a dress that's not too flashy.
The important thing is to look neat and put together, but still comfortable and not too formal. So, you can still have fun with your clothes but also show that you're taking things seriously, like going to work or meeting important people.